Modern wind turbine rotor blades are built from fiber-reinforced composites combined with core members, such as balsa wood or plastic foam.
For example, EP 2 123 431 A1 describes a method for manufacturing a rotor blade using a vacuum-assisted resin transfer molding (VARTM)-process. In a first step of the manufacturing process, fiber material is laid onto a lower part and an upper part of a mold, respectively. The fiber material is secured in place by vacuum applied from beneath. Then, mold cores are covered in vacuum bags and are placed in the lower part of the mold together with a web (also known as a shear web). Next, the upper part of the mold, together with the fiber material is turned 180° about its longitudinal axis and put into place so that the mold is closed. In a further step, a vacuum is applied to the space between the mold cores and the mold. Then, resin is injected. When the resin has set, the mold is opened and the cured blade is removed from the mold.
Since the fiber material has a much higher stiffness than the cured resin, it is intended to achieve the highest possible fiber content. A typical E-modulus of the resin is 3 GPa and a typical E-modulus for a glass fiber material is 75 GPa or more. The E-modulus for carbon fibers can be, for example, 240 GPa.
It is well known that unidirectional (UD) laminates have a lower resistance to transverse loads. This can be attributed to bonding failures at the interface between the resin and the fiber material when the stress direction is perpendicular to the fiber surface.
To improve the resistance to transverse loads of unidirectional laminates (in final use or during handling and transportation), it is known to reinforce these with transverse stabilizing fibers. When the unidirectional laminate is loaded in its main direction (corresponding to the lengthwise direction of the fibers), the transverse stabilizing fibers are loaded perpendicularly to their lengthwise axes.
It has been discovered that fatigue damages in these kinds of laminates are regularly initiated by bonding failures at the interface between the resin and the transverse stabilizing fibers.